Adolescent Nicotine Exposure Alters GABAA Receptor Signaling in the Ventral Tegmental Area and Increases Adult Ethanol Self-Administration

Dynamic Changes in Chloride Homeostasis Coordinate Midbrain Inhibitory Network Activity during Reward Learning

The ability to associate environmental stimuli with positive outcomes is a fundamental form of learning. While extensive research has focused on the response profiles of midbrain dopamine neurons during associative learning, less is known about learning-mediated changes in the afferents that shape their responses. We demonstrate that during critical phases of learning, anion homeostasis in midbrain GABA neurons - a primary source of input to dopamine neurons - is disrupted due to downregulation of the chloride transporter KCC2. This alteration in GABA neurons preferentially impacted lateral mesoaccumbal dopamine pathways and was not observed after learning was established. At the network level, learning-mediated KCC2 downregulation was associated with enhanced synchronization between individual GABA neurons and increased dopamine responses to reward-related stimuli. Conversely, enhancing KCC2 function during learning reduced GABA synchronization, diminished relevant dopamine signaling, and prevented cue-reward associations. Thus, circuit-specific adaptations in midbrain GABA neurons are crucial for forming new reward-related behaviors.

Prepubescent Electronic Cigarette Exposure Affects Sexual Motivation and Puberty in Female But Not Male Long-Evans Rats

INTRODUCTION

A method for delivering vaporized nicotine to animals has been developed using e-cigarette devices. The present experiment was designed to measure the effects of e-cigarette nicotine on pubertal onset and development of reproductive behavior in female and male Long-Evans rats.

AIM AND METHODS

Rats received daily 10-min sessions of electronic-cigarette vaporized nicotine (5% Virginia Tobacco JUUL Pods) or room air in a whole-body exposure chamber (postnatal day 28-31). Pubertal onset was monitored daily (ie, vaginal opening in females, preputial separation in males). Two weeks later, rats were tested for sexual motivation using the partner-preference paradigm, whereby subjects were given the opportunity to approach either a sexual partner or a same-sex social partner. Four weeks later, partner preference was assessed again, 10 min after rats were re-exposed to their same prepubertal treatment.

RESULTS

We found that prepubescent electronic-cigarette vaporized nicotine disrupted puberty and sexual motivation in female but not male rats. In vaped females, vaginal opening was delayed and less time was spent with the male stimulus compared to room-air controls. In contrast, no effect of e-cigarette vapor was observed on pubertal onset or on any measures of sexual behavior in male rats. No effects were observed in either female or male rats on the second partner-preference test.

CONCLUSIONS

Prepubescent vaporized nicotine affected the development of reproductive physiology and behavior in female rats but not in male rats, whereas an additional acute exposure to nicotine vapor had no effect in either female or male adult rats.

IMPLICATIONS

Given the prevalence of increasingly younger users, more animal research is needed to explore the effects of e-cigarette smoking on multiple developmental systems including reproductive physiology and behavior. This model could be useful in exploring multiple behavioral and physiological endpoints in both sexes. Adjustments to the duration of exposure and control conditions will be necessary for future experiments to best model human use.

Midbrain KCC2 downregulation: Implications for stress-related and substance use behaviors

Stress-related and substance use disorders are both characterized by disruptions in reward-related behaviors, and these disorders are often comorbid with one another. Recent investigations have identified a novel mechanism of inhibitory plasticity induced by both stress and substance use within the ventral tegmental area (VTA), a key region in reward processing. This mechanism involves the neuron-specific potassium chloride cotransporter isoform 2 (KCC2), which is essential in modulating inhibitory signaling through the regulation of intracellular chloride (Cl-) in VTA GABA neurons. Experiences, such as exposure to stress or substance use, diminish KCC2 expression in VTA GABA neurons, leading to abnormal reward-related behaviors. Here, we review literature suggesting that KCC2 downregulation contributes to irregular dopamine (DA) transmission, impacting multiple reward circuits and promoting maladaptive behaviors. Activating KCC2 restores canonical GABA functioning and reduces behavioral deficits in preclinical models, leading us to advocate for KCC2 as a target for therapies aimed at alleviating and mitigating various stress-related and substance use disorders.

Neurological Disorders Induced by Drug Use: Effects of Adolescent and Embryonic Drug Exposure on Behavioral Neurodevelopment

Clinical studies demonstrate that the risk of developing neurological disorders is increased by overconsumption of the commonly used drugs, alcohol, nicotine and cannabis. These drug-induced neurological disorders, which include substance use disorder (SUD) and its co-occurring emotional conditions such as anxiety and depression, are observed not only in adults but also with drug use during adolescence and after prenatal exposure to these drugs, and they are accompanied by long-lasting disturbances in brain development. This report provides overviews of clinical and preclinical studies, which confirm these adverse effects in adolescents and the offspring prenatally exposed to the drugs and include a more in-depth description of specific neuronal systems, their neurocircuitry and molecular mechanisms, affected by drug exposure and of specific techniques used to determine if these effects in the brain are causally related to the behavioral disturbances. With analysis of further studies, this review then addresses four specific questions that are important for fully understanding the impact that drug use in young individuals can have on future pregnancies and their offspring. Evidence demonstrates that the adverse effects on their brain and behavior can occur: (1) at low doses with short periods of drug exposure during pregnancy; (2) after pre-conception drug use by both females and males; (3) in subsequent generations following the initial drug exposure; and (4) in a sex-dependent manner, with drug use producing a greater risk in females than males of developing SUDs with emotional conditions and female offspring after prenatal drug exposure responding more adversely than male offspring. With the recent rise in drug use by adolescents and pregnant women that has occurred in association with the legalization of cannabis and increased availability of vaping tools, these conclusions from the clinical and preclinical literature are particularly alarming and underscore the urgent need to educate young women and men about the possible harmful effects of early drug use and to seek novel therapeutic strategies that might help to limit drug use in young individuals.

A paradoxical switch: the implications of excitatory GABAergic signaling in neurological disorders

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. In the mature brain, inhibitory GABAergic signaling is critical in maintaining neuronal homeostasis and vital human behaviors such as cognition, emotion, and motivation. While classically known to inhibit neuronal function under physiological conditions, previous research indicates a paradoxical switch from inhibitory to excitatory GABAergic signaling that is implicated in several neurological disorders. Various mechanisms have been proposed to contribute to the excitatory switch such as chloride ion dyshomeostasis, alterations in inhibitory receptor expression, and modifications in GABAergic synaptic plasticity. Of note, the hypothesized mechanisms underlying excitatory GABAergic signaling are highlighted in a number of neurodevelopmental, substance use, stress, and neurodegenerative disorders. Herein, we present an updated review discussing the presence of excitatory GABAergic signaling in various neurological disorders, and their potential contributions towards disease pathology.

Subregion specific neuroadaptations in the female rat striatum during acute and protracted withdrawal from nicotine

Epidemiological studies and clinical observations suggest that nicotine, a major contributor of the global burden of disease, acts in a partially sex specific manner. Still, preclinical research has primarily been conducted in males. More research is thus required to define the effects displayed by nicotine on the female brain. To this end, female rats received 15 injections of either nicotine (0.36mg/kg) or saline, over a 3-week period and were then followed for up to 3 months. Behavioral effects of nicotine were assessed using locomotor activity measurements and elevated plus maze, while neurophysiological changes were monitored using ex vivo electrophysiological field potential recordings conducted in subregions of the dorsal and ventral striatum. Behavioral assessments demonstrated a robust sensitization to the locomotor stimulatory properties of nicotine, but monitored behaviors on the elevated plus maze were not affected during acute (24 h) or protracted (3 months) withdrawal. Electrophysiological recordings revealed a selective increase in excitatory neurotransmission in the nucleus accumbens shell and dorsomedial striatum during acute withdrawal. Importantly, accumbal neuroadaptations in nicotine-treated rats correlated with locomotor behavior, supporting a role for the nucleus accumbens in behavioral sensitization. While no sustained neuroadaptations were observed following 3 months withdrawal, there was an overall trend towards reduced inhibitory tone. Together, these findings suggest that nicotine produces selective transformations of striatal brain circuits that may drive specific behaviors associated with nicotine exposure. Furthermore, our observations suggest that sex-specificity should be considered when evaluating long-term effects by nicotine on the brain.

Adolescent alcohol and nicotine exposure alters the adult response to alcohol use

Adolescence through young adulthood is a unique period of neuronal development and maturation. Numerous agents can alter this process, resulting in long-term neurological and biological consequences. In the clinical literature, it is frequently reported that adolescent alcohol consumption increases the propensity to develop addictions, including alcohol use disorder (AUD), during adulthood. A general limitation of both clinical and human pre-clinical adolescent alcohol research is the high rate of co-using/abusing more than one drug during adolescence, such as co-using/abusing alcohol with nicotine. A primary goal of basic research is elucidating neuroadaptations produced by adolescent alcohol exposure/consumption that promote alcohol and other drug self-administration in adulthood. The long-term goal is to develop pharmacotherapeutics for the prevention or amelioration of these neuroadaptations. This review will focus on studies that have examined the effects of adolescent alcohol and nicotine exposure on adult alcohol consumption, the hypersensitivity of the mesolimbic dopaminergic system, and enhanced responses not only to alcohol but also to nicotine during adulthood. Again, the long-term goal is to identify potential cholinergic agents to prevent or ameliorate the consequences of, peri-adolescent alcohol abuse.

Sex differences in VTA GABA transmission and plasticity during opioid withdrawal

The effectiveness of current treatments for opioid use disorder (OUD) varies by sex. Our understanding of the neurobiological mechanisms mediating negative states during withdrawal is lacking, particularly with regard to sex differences. Based on preclinical research in male subjects, opioid withdrawal is accompanied by increased gamma-aminobutyric acid (GABA) release probability at synapses onto dopamine neurons in the ventral tegmental area (VTA). It is unclear, however, if the physiological consequences of morphine that were originally elucidated in male rodents extend to females. The effects of morphine on the induction of future synaptic plasticity are also unknown. Here, we show that inhibitory synaptic long-term potentiation (LTP_GABA) is occluded in the VTA in male mice after repeated morphine injections and 1 day of withdrawal, while morphine-treated female mice maintain the ability to evoke LTP_GABA and have basal GABA activity similar to controls. Our observation of this physiological difference between male and female mice connects previous reports of sex differences in areas upstream and downstream of the GABA-dopamine synapse in the VTA during opioid withdrawal. The sex differences highlight the mechanistic distinctions between males and females that can be targeted when designing and implementing treatments for OUD.

Intergenerational Perioperative Neurocognitive Disorder

Accelerated neurocognitive decline after general anesthesia/surgery, also known as perioperative neurocognitive disorder (PND), is a widely recognized public health problem that may affect millions of patients each year. Advanced age, with its increasing prevalence of heightened stress, inflammation, and neurodegenerative alterations, is a consistent contributing factor to the development of PND. Although a strong homeostatic reserve in young adults makes them more resilient to PND, animal data suggest that young adults with pathophysiological conditions characterized by excessive stress and inflammation may be vulnerable to PND, and this altered phenotype may be passed to future offspring (intergenerational PND). The purpose of this narrative review of data in the literature and the authors' own experimental findings in rodents is to draw attention to the possibility of intergenerational PND, a new phenomenon which, if confirmed in humans, may unravel a big new population that may be affected by parental PND. In particular, we discuss the roles of stress, inflammation, and epigenetic alterations in the development of PND. We also discuss experimental findings that demonstrate the effects of surgery, traumatic brain injury, and the general anesthetic sevoflurane that interact to induce persistent dysregulation of the stress response system, inflammation markers, and behavior in young adult male rats and in their future offspring who have neither trauma nor anesthetic exposure (i.e., an animal model of intergenerational PND).

Role of KCC2 in the Regulation of Brain-Derived Neurotrophic Factor on Ethanol Consumption in Rats

Alcohol use disorder (AUD) is a common and complex disorder resulting from repetitive alcohol drinking. The mesocorticolimbic dopamine (DA) system, originating from the ventral tegmental area (VTA) in the midbrain, is involved in the rewarding effect of ethanol. The γ-aminobutyric acid (GABA) neurons in VTA appear to be key substrates of acute and chronic ethanol, which regulates DA neurotransmission indirectly in the mesocorticolimbic system. Despite significant research on the relationship between brain-derived neurotrophic factor (BDNF) and reduced alcohol consumption in male rats involving tropomyosin-related kinase B (TrkB), the mechanisms of BDNF-TrkB regulating alcohol behavior remain scarce. K⁺-Cl^- cotransporter 2 (KCC2) plays a crucial role in synaptic function in GABAergic neurons by modulating intracellular chlorine homeostasis. Here, we found that 4-week intermittent alcohol exposure impaired the function of KCC2 in VTA, evidenced by a lower expression level of phosphorylated KCC2 and decreased ratio of phosphorylated KCC2 to total KCC2, especially 72 h after withdrawal from 4-week ethanol exposure in male rats. CLP290 (a KCC2 activator) reduced excessive alcohol consumption after alcohol withdrawal, whereas VU0240551 (a specific KCC2 inhibitor) further enhanced alcohol intake. Importantly, VU0240551 reversed the attenuating effects of BDNF and 7,8-dihydroxyflavone (7,8-DHF) on alcohol consumption after withdrawal. Moreover, intraperitoneal injection of 7,8-DHF upregulated KCC2 expression and phosphorylated KCC2 in VTA 72 h after withdrawal from ethanol exposure in male rats. Collectively, our data indicate that KCC2 may be critical in the regulating action of BDNF-TrkB on ethanol consumption in AUD.

Ionic Plasticity: Common Mechanistic Underpinnings of Pathology in Spinal Cord Injury and the Brain

The neurotransmitter GABA is normally characterized as having an inhibitory effect on neural activity in the adult central nervous system (CNS), which quells over-excitation and limits neural plasticity. Spinal cord injury (SCI) can bring about a modification that weakens the inhibitory effect of GABA in the central gray caudal to injury. This change is linked to the downregulation of the potassium/chloride cotransporter (KCC2) and the consequent rise in intracellular Cl- in the postsynaptic neuron. As the intracellular concentration increases, the inward flow of Cl- through an ionotropic GABA-A receptor is reduced, which decreases its hyperpolarizing (inhibitory) effect, a modulatory effect known as ionic plasticity. The loss of GABA-dependent inhibition enables a state of over-excitation within the spinal cord that fosters aberrant motor activity (spasticity) and chronic pain. A downregulation of KCC2 also contributes to the development of a number of brain-dependent pathologies linked to states of neural over-excitation, including epilepsy, addiction, and developmental disorders, along with other diseases such as hypertension, asthma, and irritable bowel syndrome. Pharmacological treatments that target ionic plasticity have been shown to bring therapeutic benefits.

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

The serotonin 2A receptor agonist TCB-2 attenuates heavy alcohol drinking and alcohol-induced midbrain inhibitory plasticity

Disruption of neuronal chloride ion (Cl^- ) homeostasis has been linked to several pathological conditions, including substance use disorder, yet targeted pharmacotherapies are lacking. In this study, we explored the potential of serotonin 2A receptor (5-HT_2A R) agonism to reduce alcohol consumption in male wild-type C57Bl/6J mice and to ameliorate alcohol-induced inhibitory plasticity in the midbrain. We found that administration of the putative 5-HT_2A R agonist TCB-2 attenuated alcohol consumption and preference but did not alter water or saccharin consumption. We hypothesized that the selective behavioural effects of TCB-2 on alcohol drinking were due, at least in part, to effects of the agonist on ventral tegmental area (VTA) neurocircuitry. Alcohol consumption impairs Cl^- transport in VTA GABA neurons, which acts as a molecular adaptation leading to increased alcohol self-administration. Using ex vivo electrophysiological recordings, we found that exposure to either intermittent volitional alcohol drinking or an acute alcohol injection diminished homeostatic Cl^- transport in VTA GABA neurons. Critically, in vivo TCB-2 administration normalized Cl^- transport in the VTA after alcohol exposure. Thus, we show a potent effect of alcohol consumption on VTA inhibitory circuitry, in the form of dysregulated Cl^- homeostasis that is reversible with agonism of 5-HT_2A Rs. Our results provide insight into the potential therapeutic action of 5-HT_2A R agonists for alcohol abuse.

Unique effects of nicotine across the lifespan

Smoking remains the leading cause of preventable death in the United States. Although combustible cigarettes are largely being replaced by tobacco-free products, nicotine use continues to increase in vulnerable populations, including youth, adolescents, and pregnant women. Nicotine exerts unique effects on specific brain regions during distinct developmental periods due to the dynamic expression of nicotinic acetylcholine receptors (nAChRs) throughout the lifespan. Nicotine exposure is a health concern not only for adults but also has neurotoxic effects on the fetus, newborn, child, and adolescent. In this review, we aim to highlight the dynamic roles of nAChRs throughout gestation, adolescence, and adulthood. We also provide clinical and preclinical evidence of the neurodevelopmental, cognitive, and behavioral consequences of nicotine exposure at different developmental periods. This comprehensive review highlights unique effects of nicotine throughout the lifespan to help elucidate interventions and public health measures to protect sensitive populations from nicotine exposure.

Maternal Urinary Cotinine Concentrations During Pregnancy Predict Infant BMI Trajectory After Birth: Analysis of 89617 Mother-Infant Pairs in the Japan Environment and Children's Study

BACKGROUND

Clinical or epidemiological conclusions remain undecided on the direct effects of active and second-hand smoking during pregnancy on childhood obesity. Urinary cotinine (UC) concentration, an accurate and quantitative marker for smoking, may elucidate the dose-dependent relationship between smoking during pregnancy and childhood obesity. To analyze the relationship between UC concentration and smoking questionnaire (SQ) classes for active and second-hand smoking in pregnant mothers and trajectory of infant Kaup index (body mass index: BMI).

METHODS

This multicenter prospective cohort study was conducted using a list-wise complete set of 35829 among 89617 mother-infant singleton pairs, recruited between 2011 and 2014, in the Japan Environment and Children's Study (JECS). Pairs were categorized according to UC levels (1 to 4 classes) or SQ (0 to 4 classes).

RESULTS

Maternal BMI at delivery was the highest in UC class 4 (highest). Maternal and paternal education of ≥16 years and annual household income were lowest in UC class 4. Infant BMI was lower at birth, but trends in BMI and ΔBMI were higher from six to 36 months step-wise in the UC classes. The above tendency was observed in the list-wise complete dataset but was emphasized after multiple imputations and corrections of cofounders. UC concentration in five SQ classes largely fluctuated, and the relationship between SQ classes and trends in BMI and ΔBMI was not statistically significant.

CONCLUSION

Infants from high UC mothers had a low BMI at birth, increasing from six to 36 months of age. UC concentrations, but not smoking questionnaire classes, predict infant BMI trajectory, suggesting that active and second-hand smoking affect child obesity in a dose-dependent manner.

The impact of adolescent nicotine exposure on alcohol use during adulthood: The role of neuropeptides

Nicotine and alcohol abuse and co-dependence represent major public health crises. Indeed, previous research has shown that the prevalence of alcoholism is higher in smokers than in non-smokers. Adolescence is a susceptible period of life for the initiation of nicotine and alcohol use and the development of nicotine-alcohol codependence. However, there is a limited number of pharmacotherapeutic agents to treat addiction to nicotine or alcohol alone. Notably, there is no effective medication to treat this comorbid disorder. This chapter aims to review the early nicotine use and its impact on subsequent alcohol abuse during adolescence and adulthood as well as the role of neuropeptides in this comorbid disorder. The preclinical and clinical findings discussed in this chapter will advance our understanding of this comorbid disorder's neurobiology and lay a foundation for developing novel pharmacotherapies to treat nicotine and alcohol codependence.

The potential role of stress and sex steroids in heritable effects of sevoflurane

Most surgical procedures require general anesthesia, which is a reversible deep sedation state lacking all perception. The induction of this state is possible because of complex molecular and neuronal network actions of general anesthetics (GAs) and other pharmacological agents. Laboratory and clinical studies indicate that the effects of GAs may not be completely reversible upon anesthesia withdrawal. The long-term neurocognitive effects of GAs, especially when administered at the extremes of ages, are an increasingly recognized health concern and the subject of extensive laboratory and clinical research. Initial studies in rodents suggest that the adverse effects of GAs, whose actions involve enhancement of GABA type A receptor activity (GABAergic GAs), can also extend to future unexposed offspring. Importantly, experimental findings show that GABAergic GAs may induce heritable effects when administered from the early postnatal period to at least young adulthood, covering nearly all age groups that may have children after exposure to anesthesia. More studies are needed to understand when and how the clinical use of GAs in a large and growing population of patients can result in lower resilience to diseases in the even larger population of their unexposed offspring. This minireview is focused on the authors' published results and data in the literature supporting the notion that GABAergic GAs, in particular sevoflurane, may upregulate systemic levels of stress and sex steroids and alter expressions of genes that are essential for the functioning of these steroid systems. The authors hypothesize that stress and sex steroids are involved in the mediation of sex-specific heritable effects of sevoflurane.

Enhancing KCC2 activity decreases hyperreflexia and spasticity after chronic spinal cord injury

After spinal cord injury (SCI), the majority of individuals develop spasticity, a debilitating condition involving involuntary movements, co-contraction of antagonistic muscles, and hyperreflexia. By acting on GABAergic and Ca2+-dependent signaling, current anti-spastic medications lead to serious side effects, including a drastic decrease in motoneuronal excitability which impairs motor function and rehabilitation efforts. Exercise, in contrast, decreases spastic symptoms without decreasing motoneuron excitability. These functional improvements coincide with an increase in expression of the chloride co-transporter KCC2 in lumbar motoneurons. Thus, we hypothesized that spastic symptoms can be alleviated directly through restoration of chloride homeostasis and endogenous inhibition by increasing KCC2 activity. Here, we used the recently developed KCC2 enhancer, CLP257, to evaluate the effects of acutely increasing KCC2 extrusion capability on spastic symptoms after chronic SCI. Sprague Dawley rats received a spinal cord transection at T12 and were either bike-trained or remained sedentary for 5 weeks. Increasing KCC2 activity in the lumbar enlargement improved the rate-dependent depression of the H-reflex and reduced both phasic and tonic EMG responses to muscle stretch in sedentary animals after chronic SCI. Furthermore, the improvements due to this pharmacological treatment mirror those of exercise. Together, our results suggest that pharmacologically increasing KCC2 activity is a promising approach to decrease spastic symptoms in individuals with SCI. By acting to directly restore endogenous inhibition, this strategy has potential to avoid severe side effects and improve the quality of life of affected individuals.

CLP290 promotes the sedative effects of midazolam in neonatal rats in a KCC2-dependent manner: A laboratory study in rats

Immature neurons dominantly express the Na+-K+-2Cl- cotransporter isoform 1 (NKCC1) rather than the K+-Cl- cotransporter isoform 2 (KCC2). The intracellular chloride ion concentration ([Cl-]i) is higher in immature neurons than in mature neurons; therefore, γ-aminobutyric acid type A (GABAA) receptor activation in immature neurons does not cause chloride ion influx and subsequent hyperpolarization. In our previous work, we found that midazolam, benzodiazepine receptor agonist, causes less sedation in neonatal rats compared to adult rats and that NKCC1 blockade by bumetanide enhances the midazolam-induced sedation in neonatal, but not in adult, rats. These results suggest that GABA receptor activation requires the predominance of KCC2 over NKCC1 to exert sedative effects. In this study, we focused on CLP290, a novel KCC2-selective activator, and found that midazolam administration at 20 mg/kg after oral CLP290 intake significantly prolonged the righting reflex latency even in neonatal rats at postnatal day 7. By contrast, CLP290 alone did not exert sedative effects. Immunohistochemistry showed that midazolam combined with CLP290 decreased the number of phosphorylated cAMP response element-binding protein-positive cells in the cerebral cortex, suggesting that CLP290 reverted the inhibitory effect of midazolam. Moreover, the sedative effect of combined CLP290 and midazolam treatment was inhibited by the administration of the KCC2-selective inhibitor VU0463271, suggesting indirectly that the sedation-promoting effect of CLP290 was mediated by KCC2 activation. To our knowledge, this study is the first report showing the sedation-promoting effect of CLP290 in neonates and providing behavioral and histological evidence that CLP290 reverted the sedative effect of GABAergic drugs through the activation of KCC2. Our data suggest that the clinical application of CLP290 may provide a breakthrough in terms of midazolam-resistant sedation.

Drug addiction co-morbidity with alcohol: Neurobiological insights

Addiction is a chronic disorder that consists of a three-stage cycle of binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. These stages involve, respectively, neuroadaptations in brain circuits involved in incentive salience and habit formation, stress surfeit and reward deficit, and executive function. Much research on addiction focuses on the neurobiology underlying single drug use. However, alcohol use disorder (AUD) can be co-morbid with substance use disorder (SUD), called dual dependence. The limited epidemiological data on dual dependence indicates that there is a large population of individuals suffering from addiction who are dependent on more than one drug and/or alcohol, yet dual dependence remains understudied in addiction research. Here, we review neurobiological data on neurotransmitter and neuropeptide systems that are known to contribute to addiction pathology and how the involvement of these systems is consistent or divergent across drug classes. In particular, we highlight the dopamine, opioid, corticotropin-releasing factor, norepinephrine, hypocretin/orexin, glucocorticoid, neuroimmune signaling, endocannabinoid, glutamate, and GABA systems. We also discuss the limited research on these systems in dual dependence. Collectively, these studies demonstrate that the use of multiple drugs can produce neuroadaptations that are distinct from single drug use. Further investigation into the neurobiology of dual dependence is necessary to develop effective treatments for addiction to multiple drugs.

Early adolescent subchronic low-dose nicotine exposure increases subsequent cocaine and fentanyl self-administration in Sprague-Dawley rats

An exponential rise in nicotine-containing electronic-cigarette use has been observed during the period of adolescence. Preclinical studies have shown that nicotine exposure during early adolescence, but not adulthood, increases subsequent drug intake and reward. Although growing clinical trends highlight that stimulant use disorders are associated with the opioid epidemic, very few studies have assessed the effects of adolescent nicotine exposure on opioid intake. The objective of our current study is to develop a new animal model to assess the causal relationship of adolescent nicotine exposure on subsequent opioid intake. In this effort, we first replicate previous studies using a well-established 4-day nicotine paradigm. Rats are pretreated with a low dose of nicotine (2 × , 30 μg/kg/0.1 mL, intravenous) or saline during early adolescence (postnatal days 28-31) or adulthood (postnatal days 86-89). Following nicotine pretreatment on postnatal day 32 or postnatal day 90, animals underwent operant intravenous self-administration for the psychostimulant, cocaine [500 μg/kg/infusion (inf)] or the opioid, fentanyl (2.5 μg/kg/inf). We successfully show that adolescent but not adult, nicotine exposure enhances cocaine self-administration in male rats. Furthermore, we illustrate early adolescent but not adult nicotine exposure enhances fentanyl self-administration, independent of sex. Overall, our findings highlight that adolescence is a unique period of development that is vulnerable to nicotine-induced enhancement for cocaine and fentanyl self-administration in rats.

Targeting the Opioid Receptors: A Promising Therapeutic Avenue for Treatment in “Heavy Drinking Smokers”

Aims

Despite a general decline in tobacco use in the last decades, the prevalence of tobacco smoking in individuals with alcohol use disorder (AUD) remains substantial (45–50%). Importantly, the co-use of both substances potentiates the adverse effects, making it a significant public health problem. Substantial evidence suggests that AUD and Tobacco use disorder (TUD) may share common mechanisms. Targeting these mechanisms may therefore provide more effective therapy. Numerous studies describe a potential role of the endogenous opioid system in both AUD and TUD. Reviewing this literature, we aim to evaluate the efficacy of molecules that target the opioid system as promising therapeutic interventions for treating alcohol and tobacco co-use disorders.

Methods

We provide a synthesis of the current epidemiological knowledge of alcohol and tobacco co-use disorders. We evaluate clinical and preclinical research that focuses on the regulation of the endogenous opioid system in alcohol, nicotine, and their interactions.

Results

The epidemiological data confirm that smoking stimulates heavy drinking and facilitates alcohol craving. Pharmacological findings suggest that treatments that are efficacious in the dual addiction provide a beneficial treatment outcome in comorbid AUD and TUD. In this regard, MOP, DOP and NOP-receptor antagonists show promising results, while the findings prompt caution when considering KOP-receptor antagonists as a treatment option in alcohol and tobacco co-use disorders.

Conclusions

Existing literature suggests a role of the opioid system in sustaining the high comorbidity rates of AUD and TUD. Molecules targeting opioid receptors may therefore represent promising therapeutic interventions in ‘heavy drinking smokers.’

Nicotinic acetylcholine receptors and nicotine addiction: A brief introduction

Nicotine is a highly addictive drug found in tobacco that drives its continued use despite the harmful consequences. The initiation of nicotine abuse involves the mesolimbic dopamine system, which contributes to the rewarding sensory stimuli and associative learning processes in the beginning stages of addiction. Nicotine binds to neuronal nicotinic acetylcholine receptors (nAChRs), which come in a diverse collection of subtypes. The nAChRs that contain the α4 and β2 subunits, often in combination with the α6 subunit, are particularly important for nicotine's ability to increase midbrain dopamine neuron firing rates and phasic burst firing. Chronic nicotine exposure results in numerous neuroadaptations, including the upregulation of particular nAChR subtypes associated with long-term desensitization of the receptors. When nicotine is no longer present, for example during attempts to quit smoking, a withdrawal syndrome develops. The expression of physical withdrawal symptoms depends mainly on the α2, α3, α5, and β4 nicotinic subunits in the epithalamic habenular complex and its target regions. Thus, nicotine affects diverse neural systems and an array of nAChR subtypes to mediate the overall addiction process. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Unique, long-term effects of nicotine on adolescent brain

Adolescence is a time of major plasticity of brain systems that regulate motivated behavior and cognition, and is also the age of peak onset of nicotine use. Although there has been a decline in teen use of cigarettes in recent years, there has been a huge increase in nicotine vaping. It is therefore critically important to understand the impact of nicotine on this critical phase of brain development. Animal studies have shown that nicotine has unique effects on adolescent brain. The goal of this review is therefore to systematically evaluate age- and sex-differences in the effects of nicotine on brain and behavior. Both acute and chronic effects of nicotine on brain biochemistry and behavior, particularly drug reward, aversion, cognition and emotion, are evaluated. Gaps in our current knowledge that need to be addressed are also highlighted. This review compares and integrates human and animals findings. Although there can be no experimental studies in humans to confirm similar behavioral effects of teen nicotine exposure, an emerging observational literature suggests similarities across species. Given the substantial evidence for long-term negative impact of adolescent nicotine exposure on brain and behavior, further longitudinal assessment of health outcomes in teen and young adult e-cigarette users is warranted.

Differential patterns of alcohol and nicotine intake: Combined alcohol and nicotine binge consumption behaviors in mice

Late adolescence and young adulthood, corresponding to the high school and college years, are vulnerable periods for increased alcohol and nicotine use. The dramatic increase in the prevalence of electronic cigarette use is particularly concerning in these age groups. Late adolescents and young adults are more likely to engage in cycles of binge drug consumption, and alcohol and nicotine are frequently used together. However, there are few data examining the combination of alcohol and nicotine in binge models in animal models. In this study, our objectives were to determine how voluntary nicotine consumption beginning in late adolescence influenced subsequent binge alcohol consumption in young adulthood, how a combination of alcohol and nicotine binge consumption differed from alcohol-only binge consumption, and whether nicotine would be consumed when presented in a binge procedure. Male C57BL/6J mice voluntarily consumed unsweetened alcohol and nicotine in continuous-access bottle-choice procedures in combination with cycles of drinking-in-the-dark. Our results show that experience with voluntary nicotine consumption in late adolescence did not affect subsequent binge alcohol consumption in early adulthood. However, mice that consumed nicotine in adolescence showed an initial decrease in alcohol preference, and consequently increase in nicotine preference, on the first session of combined ethanol and nicotine binge consumption in adulthood compared with mice that drank only water during late adolescence. Lastly, we found that mice readily consumed unsweetened nicotine when presented in a binge procedure, and the level of consumption exceeded the nicotine consumption observed in the combination alcohol and nicotine binge. Our data show that expansion of the patterns of alcohol and nicotine co-consumption in a mouse models is possible, which will enable us to dissect relevant molecular targets underlying these consumption patterns and better inform drug development efforts.

Acute Nicotine Exposure Alters Ventral Tegmental Area Inhibitory Transmission and Promotes Diazepam Consumption

Nicotine use increases the risk for subsequent abuse of other addictive drugs, but the biological basis underlying this risk remains largely unknown. Interactions between nicotine and other drugs of abuse may arise from nicotine-induced neural adaptations in the mesolimbic dopamine (DA) system, a common pathway for the reinforcing effects of many addictive substances. Previous work identified nicotine-induced neuroadaptations that alter inhibitory transmission in the ventral tegmental area (VTA). Here, we test whether nicotine-induced dysregulation of GABAergic signaling within the VTA increases the vulnerability for benzodiazepine abuse that has been reported in smokers. We demonstrate in rats that nicotine exposure dysregulates diazepam-induced inhibition of VTA GABA neurons and increases diazepam consumption. In VTA GABA neurons, nicotine impaired KCC2-mediated chloride extrusion, depolarized the GABAA reversal potential, and shifted the pharmacological effect of diazepam on GABA neurons from inhibition toward excitation. In parallel, nicotine-related alterations in GABA signaling observed ex vivo were associated with enhanced diazepam-induced inhibition of lateral VTA DA neurons in vivo Targeting KCC2 with the agonist CLP290 normalized diazepam-induced effects on VTA GABA transmission and reduced diazepam consumption following nicotine administration to the control level. Together, our results provide insights into midbrain circuit alterations resulting from nicotine exposure that contribute to the abuse of other drugs, such as benzodiazepines.

Relaxation of synaptic inhibitory events as a compensatory mechanism in fetal SOD spinal motor networks

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease affecting motor neurons (MNs) during late adulthood. Here, with the aim of identifying early changes underpinning ALS neurodegeneration, we analyzed the GABAergic/glycinergic inputs to E17.5 fetal MNs from SOD1G93A (SOD) mice in parallel with chloride homeostasis. Our results show that IPSCs are less frequent in SOD animals in accordance with a reduction of synaptic VIAAT-positive terminals. SOD MNs exhibited an EGABAAR10 mV more depolarized than in WT MNs associated with a KCC2 reduction. Interestingly, SOD GABAergic/glycinergic IPSCs and evoked GABAAR-currents exhibited a slower decay correlated to elevated [Cl-]i. Computer simulations revealed that a slower relaxation of synaptic inhibitory events acts as compensatory mechanism to strengthen GABA/glycine inhibition when EGABAAR is more depolarized. How such mechanisms evolve during pathophysiological processes remain to be determined, but our data indicate that at least SOD1 familial ALS may be considered as a neurodevelopmental disease.

5-HT2A receptor activation normalizes stress-induced dysregulation of GABAergic signaling in the ventral tegmental area

Stress is known to alter GABAergic signaling in the ventral tegmental area (VTA), and this inhibitory plasticity is associated with increased alcohol self-administration. In humans, serotonin 2A receptor (5-HT_2AR) agonists can treat stress- and alcohol-related disorders, but the neural substrates are ill-defined. Thus, we reasoned that 5-HT_2AR pharmacotherapies may ameliorate the stress-induced dysregulated inhibitory VTA circuitry that contributes to subsequent alcohol abuse. We found that acute stress exposure in mice compromised GABA-mediated inhibition of VTA GABA neurons corresponding with increased ethanol-induced GABAergic transmission. This stress-induced inhibitory plasticity was reversible by applying the 5-HT_2AR agonist TCB-2 ex vivo via functional enhancement of the potassium-chloride cotransporter KCC2. The signaling pathway linking 5-HT_2AR activation and normalization of KCC2 function was dependent on protein kinase C signaling and phosphorylation of KCC2 at serine 940 (S940), as mutation of S940 to alanine prevented restoration of chloride transport function by TCB-2. Through positive modulation of KCC2, TCB-2 also reduced elevated ethanol-induced GABAergic signaling after stress exposure that has previously been linked to increased ethanol consumption. Collectively, these findings provide mechanistic insights into the therapeutic action of 5-HT_2AR agonists at the neuronal and circuit levels of brain reward circuitry.

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.

Peri-adolescent alcohol consumption increases sensitivity and dopaminergic response to nicotine during adulthood in female alcohol-preferring (P) rats: Alterations to α7 nicotinic acetylcholine receptor expression

Adolescent alcohol drinking has been linked to increased risk for drug abuse during adulthood. Nicotine microinjected directly into the posterior ventral tegmental area (pVTA) stimulates dopamine (DA) release in the nucleus accumbens (NAc) shell. The α7 nicotinic acetylcholine receptor (nAChR) is a potent regulator of dopaminergic activity in the pVTA. The current experiments examined the effects of peri-adolescent ethanol (EtOH) drinking on the ability of intra-pVTA nicotine to stimulate DA release during adulthood and alterations in α7 nAChR expression within the pVTA. Alcohol-preferring (P) female rats consumed EtOH and/or water during adolescence (post-natal day [PND] 30-60) or adulthood (PND 90-120). Thirty days following removal of EtOH, subjects received microinjections of 1 μM, 10 μM, or 50 μM nicotine into the pVTA concurrently with microdialysis for extracellular DA in the NAc shell. Brains were harvested from an additional cohort after PND 90 for quantification of α7 nAChR within the pVTA. The results indicated that only adolescent EtOH consumption produced a leftward and upward shift in the dose response curve for nicotine to stimulate DA release in the NAc shell. Investigation of α7 nAChR expression within the pVTA revealed a significant increase in animals that consumed EtOH during adolescence compared to naïve animals. The data suggests that peri-adolescent EtOH consumption produced cross-sensitization to the effects of nicotine during adulthood. The interaction between adolescent EtOH consumption and inflated adult risk for drug dependency could be predicated, at least in part, upon alterations in α7 nAChR expression within the mesolimbic reward pathway.

Nicotine Gateway Effects on Adolescent Substance Use

Given the rise in teenage use of electronic nicotine delivery systems ("vaping") in congruence with the increasing numbers of drug-related emergencies, it is critical to expand the knowledge of the physical and behavioral risks associated with developmental nicotine exposure. A further understanding of the molecular and neurochemical underpinnings of nicotine's gateway effects allows emergency clinicians to advise patients and families and adjust treatment accordingly, which may minimize the use of tobacco, nicotine, and future substances. Currently, the growing use of tobacco products and electronic cigarettes among teenagers represents a major public health concern. Adolescent exposure to tobacco or nicotine can lead to subsequent abuse of nicotine and other substances, which is known as the gateway hypothesis. Adolescence is a developmentally sensitive time period when risk-taking behaviors, such as sensation seeking and drug experimentation, often begin. These hallmark behaviors of adolescence are largely due to maturational changes in the brain. The developing brain is particularly vulnerable to the harmful effects of drugs of abuse, including tobacco and nicotine products, which activate nicotinic acetylcholine receptors (nAChRs). Disruption of nAChR development with early nicotine use may influence the function and pharmacology of the receptor subunits and alter the release of reward-related neurotransmitters, including acetylcholine, dopamine, GABA, serotonin, and glutamate. In this review, we emphasize that the effects of nicotine are highly dependent on timing of exposure, with a dynamic interaction of nAChRs with dopaminergic, endocannabinoid, and opioidergic systems to enhance general drug reward and reinforcement. We analyzed available literature regarding adolescent substance use and nicotine's impact on the developing brain and behavior using the electronic databases of PubMed and Google Scholar for articles published in English between January 1968 and November 2018. We present a large collection of clinical and preclinical evidence that adolescent nicotine exposure influences long-term molecular, biochemical, and functional changes in the brain that encourage subsequent drug abuse.

Nicotine and alcohol: the role of midbrain dopaminergic neurons in drug reinforcement

Nicotine and alcohol addiction are leading causes of preventable death worldwide and continue to constitute a huge socio-economic burden. Both nicotine and alcohol perturb the brain's mesocorticolimbic system. Dopamine (DA) neurons projecting from the ventral tegmental area (VTA) to multiple downstream structures, including the nucleus accumbens, prefrontal cortex, and amygdala, are highly involved in the maintenance of healthy brain function. VTA DA neurons play a crucial role in associative learning and reinforcement. Nicotine and alcohol usurp these functions, promoting reinforcement of drug taking behaviors. In this review, we will first describe how nicotine and alcohol individually affect VTA DA neurons by examining how drug exposure alters the heterogeneous VTA microcircuit and network-wide projections. We will also examine how coadministration or previous exposure to nicotine or alcohol may augment the reinforcing effects of the other. Additionally, this review briefly summarizes the role of VTA DA neurons in nicotine, alcohol, and their synergistic effects in reinforcement and also addresses the remaining questions related to the circuit-function specificity of the dopaminergic system in mediating nicotine/alcohol reinforcement and comorbidity.

Neurobiological Considerations for Tobacco Use Disorder

PURPOSE OF REVIEW

Neurobiological studies of tobacco/nicotine use examining genetic, molecular, functional, and behavioral correlates have improved our understanding of nicotine/tobacco dependence and have informed treatment. Recent work extending previously established findings and reporting novel methodologies and discoveries in preclinical and human studies are reviewed.

RECENT FINDINGS

Recent work in preclinical models has focused on the differential roles of nicotinic receptor subtypes and nicotine's effects on neural systems beyond cortico-striatal dopaminergic pathways, and utilizing advanced methodologies such as pharmacogenetics, optogenetics and rodent fMRI to identify targets for treatment. Likewise, human neuroimaging studies have identified molecular and functional dynamic shifts associated with tobacco/nicotine use that further inform treatment.

SUMMARY

Nicotine/tobacco use is associated with widespread neural adaptations that are persistent and function to maintain addiction. The continued identification of genetic, molecular, neural, and behavioral endophenotypes related to nicotine/tobacco use, dependence, and addiction will facilitate the development and delivery of personalized treatment.

Inhibitory Plasticity of Mesocorticolimbic Circuits in Addiction and Mental Illness

Behavioral adaptations occur through remodeling of brain circuits, as arising, for instance, from experience-dependent synaptic plasticity. Drugs of abuse and aversive stimuli, such as stress, act on the mesocorticolimbic system, dysregulating adaptive mechanisms and leading to a variety of aberrant behaviors associated with neuropsychiatric disorders. Until recently, research in the field has commonly focused on experience-dependent synaptic plasticity at excitatory synapses. However, there is growing evidence that synaptic plasticity within inhibitory circuits is an important contributor to maladaptive behaviors. We speculate that restoring normal inhibitory synaptic transmission is a promising therapeutic target for correcting some of the circuit abnormalities underlying neuropsychiatric disorders.

Effects of combined brief etomidate anesthesia and postnatal stress on amygdala expression of Cl- cotransporters and corticotropin-releasing hormone and alcohol intake in adult rats

Early life stressors, including general anesthesia, can have adverse effects on adult neural and behavioral outcomes, such as disruptions in inhibitory signaling, stress responsivity and increased risk of psychiatric disorders. Here we used a rat model to determine the effects of combined exposure to etomidate (ET) neonatal anesthesia and maternal separation on adult amygdala expression of genes for corticotropin-releasing hormone (Crh) and the chloride co-transporters Nkcc1 and Kcc2, as well as ethanol intake. Male and female Sprague-Dawley rats were subjected to 2 h of ET anesthesia on postnatal days (P) 4, 5, or 6 followed by maternal separation for 3 h on P10 (ET + SEP). During the P91-P120 period rats had daily 2 h access to three 0.05% saccharin solutions containing 0%, 5%, or 10% ethanol, followed by gene expression analyses. The ET + SEP group had increased Crh mRNA levels and Nkcc1/Kcc2 mRNA ratios in the amygdala, with greater increases in Nkcc1/Kcc2 mRNA ratios in males. A moderate increase in 5% ethanol intake was evident in the ET + SEP males, but not females, after calculation of the ratio of alcohol intake between the last week and first week of exposure. In contrast, control males tended to decrease alcohol consumption during the same period. A brief exposure to ET combined with a subsequent episode of stress early in life induced significant alterations in expression of amygdala Crh, Nkcc1 and Kcc2 with greater changes in the Cl^- transporter expression in males. The possibility of increased alcohol intake in the exposed males requires further confirmation using different alcohol intake paradigms.